Method for transferring nanowires  from a fluid to a substrate surface

ABSTRACT

A method for transferring an assembly of oriented nanowires from a fluid to a substrate surface, comprising: providing (FIG. 2A) a fluid to a container, said fluid comprising a first liquid (11), a second liquid (12) and a plurality of nanowires (25), wherein the first and second liquids phase separate into a sub phase, a top phase, and an interface (13) between the sub phase and the top phase; wherein the nanowires are functionalized to align vertically into a nanowire aggregate at the interface; wherein the fluid is provided with a substance in a composition configured to change the composition of the top phase or the composition of the sub phase to counteract bulging of the interface (FIG. 2B); and bringing the nanowire aggregate into contact with a substrate surface such that a majority of the nanowires are aligned with respect to each other on the substrate.

FIELD OF THE INVENTION

The present invention relates to the formation of nanowire devices andin particular to capturing and aligning of nanowires to make nanowiredevices. More specifically, this disclosure is related to methods fortransferring oriented nanowires from a fluid to a substrate surface.

BACKGROUND

Conventional technologies for capturing nanostructures on a surfacefocus on the alignment and capture/deposition of nanostructures with alow length/diameter ratio (e.g.: nanorods, nanoparticles). However,capture and alignment of nanostructures with appreciable length/diameterratio (e.g. nanowires) is more difficult. It is also difficult to alignnanowires with a preferential direction. Conventional technologies useexternal controls (e.g., applied electric fields, slow solventevaporation or thermal annealing) which may require the use of externalequipment or high voltages to obtain the alignment andcapture/deposition of nanostructures. These external controls increasethe production cost and decrease the scalability of nanowire deviceproduction.

Applicant's previous application, published as WO2015/166416 A1,discloses a method for capturing and aligning an assembly of nanowiresfrom a liquid interface onto a surface including providing a firstliquid and a second liquid, wherein the first and second liquids phaseseparate into a sub phase, a top phase and an interface between the subphase and the top phase. The nanowires are provided such that themajority of the nanowires are located at the interface and providing thenanowires onto a substrate such that a majority of the nanowires arealigned with respect to each other on the substrate.

There still exists room for improvement in the arts of providing asuitable assembly of nanowires, and for capturing the assembly on asubstrate surface. More specifically, improvements are desired forobtaining higher quality in the process of transferring a nanowireaggregate from a fluid to a substrate surface, and also in theperfection level of the resulting substrate.

SUMMARY

Solutions are presented herein, which address problems related totransferring an assembly of nanowires to a substrate surface. Oneembodiment is drawn to a method for transferring an assembly of orientednanowires from a fluid to a substrate surface, comprising: providing afluid to a container, said fluid comprising a first liquid, a secondliquid and a plurality of nanowires, wherein the first and secondliquids phase separate into a sub phase, a top phase, and an interfacebetween the sub phase and the top phase; wherein the nanowires arefunctionalized to align vertically into a nanowire aggregate at theinterface; wherein said fluid is provided with a substance in acomposition configured to counteract bulging of the interface; andbringing the nanowire aggregate into contact with a substrate surfacesuch that a majority of the nanowires are aligned with respect to eachother on the substrate.

Further detailed solutions are provided in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D schematically illustrate various steps of a method ofcapturing and aligning an assembly of nanowires according to anembodiment.

FIGS. 2A and 2B schematically illustrate a phase interface including ananowire aggregate, before and after fluid substance modificationaccording to an embodiment.

FIG. 3 is a schematic illustration of a functionalized nanowireaccording to an embodiment.

FIG. 4 is a schematic illustration of an aggregate of aligned andassembled nanowires according at a fluid interface according to anembodiment.

FIG. 5 is a schematic illustration of a method of transferring anassembly of oriented nanowires from a fluid to a substrate surface.

FIG. 6 is a schematic illustration of a container apparatus fortransferring an assembly of oriented nanowires from a fluid to asubstrate surface.

FIG. 7 is a schematic side cross sectional view of a nanowire device,e.g. a solar cell, according to an embodiment.

FIG. 8 is an image showing nanowires transferred to a substrate using amethod as provided herein.

DETAILED DESCRIPTION

Various embodiments will be described below with reference to thedrawings. The embodiments are to be seen as exemplary, and other ways ofrealizing the solutions provided within the scope of the claims aretherefore foreseeable.

The invention relates generally to the forming of an aggregate of ananowire assembly of nanowires which are aligned, preferably in a commondirection, at an interface which is formed between substantiallyimmiscible first and second liquids of a fluid. This fluid containingnanowires may be formed in various ways, e.g. by first combining thefirst and second liquids and thereafter add the nanowires, or by addingthe nanowires to one of the liquids before combining it with the otherliquid. Suitably functionalized nanowires will then prone to assemble atthe interface. However, the art of capturing aligned nanowires on asubstrate surface from such a fluid interface has still been associatedwith a number of hurdles, and there is a desire in raising the qualityof such nanowire substrates, in terms of e.g. nanowire alignment,orientation and surface density. The inventors have realized that asolution for increasing this quality is to prepare the fluid compositionsuch that the nanowire aggregate is suitably formed at the interface.Furthermore, the fluid is provided with a substance in a compositionconfigured to counteract bulging of the interface. This may in variousembodiments be obtained by selectively arranging the composition of thatsubstance, such as by careful selection of the type of substance and itsconcentration. This way, the interface is stretched out, so as to besubstantially planarized for suitable engagement with a planar substratesurface. In other embodiments, the fluid composition may be modified ina subsequent step, after allowing the interface to form in the fluid, byaddition of the substance in a composition such that the interface isstretched out to be substantially planarized for suitable engagementwith a planar substrate surface. In this context, planarization aims atthe effect of minimizing or decreasing bulging or bellying of an upperphase into a lower phase, i.e. increasing the curvature radius of theinterface in a vertical plane. In the extreme case, this would meangoing from a state where the second, top phase, liquid may be suspendedas substantially spherical droplets at the surface of the first liquid11, to a state where the second liquid floats on top of the first, subphase, liquid with a substantially horizontal interface. Variousembodiments and aspects will now be described.

With reference to FIG. 1A, an embodiment of the method uses a firstliquid 11 located in a container 1 and a nanowire dispersion 12,constituting a second liquid 12, that is added to the first liquid 11.The nanowire dispersion 12 is preferably made by dispersingprefabricated nanowires 20 in a dispersion liquid. That is, thenanowires are fabricated prior to being added to the dispersion fluid 12in contrast to in-situ formed nanowires in the dispersion fluid. In thisembodiment, the dispersion liquid 12 is selected such that the nanowiredispersion is immiscible or only partially miscible in the first liquid11. In this manner, when the nanowire dispersion is added to the firstliquid 11, the first liquid 11 and the dispersion liquid 12 phaseseparate, creating a two phase liquid system. The denser liquid 11settles to the bottom of the container 1 forming a sub phase, while theless dense liquid 12 floats on top of the first liquid 11 creating a topphase. The resulting two-phase system has a top phase and a sub phaseand an interface 13 between the top phase and the sub phase.

In an embodiment, the nanowires 20 in the nanowire dispersion may bemade of the same material. Alternatively, the nanowire dispersion 12 mayinclude nanowires made of different materials. Nanowire materialssuitable for use in the present embodiment and the embodiments belowinclude metals (such as gold silver and alloys thereof), carbonnanowires or nanotubes (single wall and multiwall), semiconductors,including III-V (including binary, ternary and quaternary III-Vsemiconductors made of Al, In, Ga, N, P, As, such as GaAs and InP) andII-VI semiconductors (including binary, ternary and quaternary II-VIsemiconductors made from Zn, Cd, Se, O, S, Te, such as ZnO, CdSe) andceramics. The nanowires 20 may be used as received or be subjected toone or more surface treatments described in more detail below.

After adding the nanowire dispersion to the first liquid 11, themajority of the nanowires are then assembled at the interface.Typically, the nanowires spontaneously assemble at the interface, thatis, the nanowires self-align at the interface, if given sufficient time.However, the nanowires may be subjected to one or more conditions thatpromote or accelerate the assembly of the nanowires at the interface.Acceleration may be accomplished in several ways. For example,acceleration may be accomplished by changing the composition of the topphase, the composition of the sub phase or altering the temperature ofthe container.

In another embodiment, rather than adding the nanowire dispersion to thefirst liquid 11, the two phase liquid system is formed first followed byadding nanowires to the system. Thus, a second liquid 12, different fromthe first liquid 11, may be added to the first liquid 11. Preferably,the second liquid 12 is immiscible or partially miscible/partiallyimmiscible in the first liquid 11. In this manner and similar to theprevious embodiment, when the second liquid 12 is added to the firstliquid 11, the two liquids phase separate, creating a two-phase liquidsystem. The denser liquid settles to the bottom of the container, whilethe less dense liquid floats on top of the first liquid 11, resulting ina two-phase system with a top phase, a sub phase and an interface 13between the top and sub phases. In this embodiment, the nanowires or ananowire dispersion may be added to the two-phase system or added to thefirst liquid 11 prior to adding the second liquid 12 to the first liquid11. A nanowire dispersion comprises nanowires distributed in adispersion liquid (e.g. a solvent). The dispersion liquid may be thesame as either the first liquid 11 or the second liquid 12.Alternatively, the dispersion liquid may be a third liquid that isdifferent from both the first and second liquids. Alternatively, drynanowires may be added.

In an embodiment, the nanowires may be functionalized with eithercompounds that render the nanowires hydrophobic (including alkanes,fluoro-compounds (such as Pentanethiol, perfluorodecane thiol,dodecyltrichlorosilane, stearic acid, decyl phosphonic acid,5-(1,2-dithiolan-3-yl)-N-dodecylpentanamide, sodium dodecyl sulfate,triphenyl phosphine, octadecylthiol)) and/or hydrophilic (includingsulphates, phosphates, carboxylates, amines, polyethers, (such as sodiummercaptopropane sulfonate, sodium mercaptoethane sulfonate,mercaptoalkane succinate (2-mercaptosuccinate), mercaptoalkane amine,(11-mercaptoundecyl)-N,N,N-trimethylammonium bromide,(12-Phosphonododecyl)phosphonic acid, (±)-1,2-Dithiolane-3-pentanoicacid, (2-Ammonioethyl)di-tert-butylphosphonium bis(tetrafluoroborate),(3-Aminopropyl)triethoxysilane, 12-mercaptododecanoic acid)).

In an embodiment, one part of the nanowire surface is renderedhydrophobic and the other part of the nanowire surface is renderedhydrophilic using different functionalizing compounds to achieve thevertical alignment at the liquid interface. In an alternativeembodiment, only one part of the nanowire surface is treated with afunctionalizing compound.

FIG. 3 is a schematic illustration of a functionalized nanowire 20according to an embodiment. As illustrated, the nanowire 20 includes ananoparticle 23, e.g. a metal particle such as a gold nanoparticle, atone end of the nanowire 20. The nanoparticle 23 may be a result of thegrowth process of the wire portion 21 (e.g. semiconductor portion) ofthe nanowire 20, such as when growing nanowires 20 by thevapor-liquid-solid (VLS) process using the nanoparticle 23 as a catalystseed. Example processes for making nanowires 20 can be found in U.S.provisional application 61/623,137 filed on Apr. 12, 2012 and PCTpublished application number WO13/154490 A2, hereby incorporated byreference in their entirety. In an embodiment, a first functionalizingcompound 24 is attached to one end of the nanowire 20, such as to thenanoparticle 23. A second, different functionalizing compound (notshown) may be attached to the second end of the nanowire that lacks thenanoparticle 23. The functionalizing compound(s) 24 aid in aligning thenanowires 20 in the top 104 and bottom 102 phases. As discussed in moredetail below in regards to specific examples, one of the functionalizinggroups may be 1-octadecanethiol (ODT), while a component attached to thelower end of the nanowire 20 may be (12-phosphonododecyl)phosphonic acid(PPA). Further, the functionalizing compound 24 may have a functionalgroup, or a different one at each ends of the functionalizing compounds24. Various types of functionalizing components and examples of suchtypes are given in Table I of applicant's own prior applicationWO2015/166416, hereby incorporated by reference in their entirety.

Based on the choice of functionalizing compounds 24 and the type andcomposition of the liquids 11, 12, the orientation and alignment of thenanowires may be controlled. In a similar manner, theseparameters/compositional variables may allow the nanowire-nanowireinterspacing to be varied, resulting in assemblies with different (i.e.pre-selected) densities (e.g. density of nanowires per square micron).The nanowire-nanowire interspacing can be deduced from the percentage ofsurface area covered after the capture of the aligned nanowires on thesubstrate.

In one embodiment, appropriate formation of a nanowire aggregate at theinterface 13 can be obtained with a dispersion of nanowires 20 in aliquid 12 comprising a component selected from the group comprising oneor more of toluene, hexane, octane, cyclohexane, cyclopentanone a thiolsuch as 1-octadecanethiol and polyethyleneimine. In a preferredembodiment, the nanowires are functionalized with a component 24 of thesecond liquid 12. The functionalizing component may comprise an atom oratoms prone to attach to the seed particle 23, e.g. sulfur, and furthera molecule chain providing hydrophobic properties to the functionalizednanowire 20.

In a preferred embodiment, functionalization is carried out in aseparate step, prior to adding the nanowire dispersion to the firstliquid 11. In one embodiment, functionalization may be carried out bymixing a solution of the functionalizing component with an added amountof nanowires. An object at this point is adsorption of thefunctionalizing component only to the nanoparticle 23, typically a goldparticle, in order to make the particle 23 highly hydrophobic,increasing in this way the Janus properties of the nanowire.Additionally, the inventors have found indications that covering themetal nanoparticle 23 with adhered molecules of the functionalizingcomponent will decrease the surface charge density of the particle,which also results in a decrease in its electrostatic repulsive force.In this way, a close-packed assembly of nanowires in the aggregate 25will be favorable.

In one embodiment, the nanowires 20 may be provided with a dielectricsurface coating 22 about the wire portion 21. The dielectric surfacecoating 22 may e.g. comprise silica or aluminum oxide. With properselection of the first liquid 11, the coated nanowire surface may besuitably wetted in the first liquid 11. This way, proper selection ofthe liquid composition of both the first liquid 11 and the second liquid12 will help promote the formation of a nanowire aggregate 25, asindicated in FIG. 4, in which all or nearly all nanowires 20 arecorrectly oriented, aligned, and densely packed. The first liquid 11 mayhave a composition comprising a concentration of first substance of atleast one of acetone, acetonitrile, dimethyl sulfoxide, di ethyleneglycol, and isopropyl alcohol. This first substance is provided to at acertain concentration in water, i.e. in an aqueous solution. Byincluding this first substance to a certain concentration in the firstliquid 11 in the sub phase the interfacial energy between the firstliquid 11 and the second liquid 12, or air, is lowered and the nanowiresurface charge density decreases. This contributes to the nanowires 20predisposition to align vertically in the interface, and thus a tighterpacking in the aggregate 25.

Once a suitable aggregate 25 of nanowires has formed at the interface13, transfer of the aggregate to a substrate surface is to be carriedout. However, due to the different characters of the liquids 11, 12,when a suitable composition for obtaining a tightly packed aggregate 25of oriented and aligned nanowires 20 have been used, the fluid may lookas in FIG. 1A. In a subsequent step, an additional amount of the secondliquid 12 may then be added in one embodiment. This way, pluralindividual aggregates 25 may be affected to merge in to a single topphase1, as shown in FIG. 1B. So, by first providing the second liquid 12in a first amount, a larger overall interface (i.e. the sum of allaggregate 25 surfaces) will alleviate the transfer of nanowires from thebulk of the second fluid to capture at the interface 13. Later additionof a second amount of the second liquid 12, or an amount with similarproperties as the second liquid 12, will assist in creating a singleaggregate 25 with a contiguous interface 13.

In one embodiment, the fluid may be provided by first providing thefirst liquid 11, and subsequently adding the second liquid 12 includinga dispersion of nanowires 20 onto the surface of the first liquid 11. Inone embodiment, this may be obtained by spraying the second fluid 12including the nanowires 20 onto the first liquid 11. In suchembodiments, a single top phase as shown in FIG. 1B or 2A may beobtained, while avoiding the droplet character of FIG. 1A.

In various embodiments, dependent on the selected properties of theliquids 11, 12, the interface 13, and hence the aggregate 25, may bulgetowards the first liquid and have a severe curvature with acomparatively small radius, as indicated in both FIGS. 1A and 1B. Thiscan also be seen in FIG. 2A, although it should be understood that thisdrawing is highly schematic, particularly with regard to the nanowiresize in relation to the container. Transfer of the nanowires to a planarsubstrate may nevertheless be obtained, e.g. by pulling a substrate atan angle from the first liquid 11. However, for larger substrates such aprocess may not be suitable and/or may not provide a high-qualityassembly of nanowires. In various embodiments, a level of quality of afilm of nanowires may correlate with absence or a low level of holes,cracks, or nanowires vertically aligned at a different height, which aretypes of defects that may be important challenges in applications suchas solar cells. According to preferred embodiments, this may be avoidedor alleviated by providing the fluid with a substance in a compositionconfigured to counteract bulging of the interface 13.

In one embodiment, this may be obtained by careful selection of thesubstance and its composition. The substance may be included in acomposition which acts to increase the relative density of the sub phasewith respect to the top phase. This may be obtained by including asubstance to the second liquid in the top phase, such that its relativedensity is below a predetermined threshold compared to the sub phase. Inan alternative embodiment, a careful configuration of the composition ofthe first liquid of the sub phase is rather obtained, by including anappropriate substance in a solvent.

In various embodiments, a step of modifying the fluid, such as modifyingits substance composition, is carried out after the aggregate has formedat the interface 13, such that a curvature radius of the interface 13 isincreased towards planarization.

In one embodiment, the modification of the fluid comprises changing therelative density relationship between the top phase and the sub phase.In one embodiment, this may be obtained by adding an auxiliary substanceto the top phase, after formation of the contiguous aggregate 25 at theinterface 13. As an example, hexane may be added to the top phase wherethe second liquid 12 is 1-octadecanethiol, such that the overall densityof the modified top phase composition is decreased.

In an alternative embodiment, modifying the substance compositionincludes changing the composition of the sub phase subsequent to formingthe nanowire aggregate. In such an embodiment, the composition of thesub phase may be changed so as to increase its density. This may beaccomplished by adding an auxiliary component or substance to the firstliquid 11 of the sub phase, so as to increase the density of the subphase.

In one embodiment, the composition of the sub phase is changed byextracting an amount of the first liquid 11 from the sub phase, andsubsequently adding an amount of liquid to the sub phase, wherein theadded amount of liquid has a different composition than the extractedamount of liquid. This process has the benefit of ensuring that thecomposition of the added amount of liquid is properly mixed in the subphase, since a prepared mixture of said different composition may beadded to the sub phase.

In one embodiment, where the substance has higher density than itssolvent, this process of changing the composition of the sub phase mayinclude providing the first liquid 11 to the sub phase with a substanceof a first concentration exceeding a first level prior to providing thenanowires to the fluid, and changing the substance concentration of thesub phase to below a second level, which is lower than the first level,after forming the nanowire aggregate. In an alternative of thisembodiment, where the substance has lower density than its solvent, theprocess of changing the composition of the sub phase may includeproviding the first liquid 11 to the sub phase with a substanceconcentration not exceeding a first level prior to providing thenanowires to the fluid, and changing the substance concentration of thesub phase to above a second level, which is at least as high as thefirst level, after forming the nanowire aggregate. These embodimentshave a benefit of not requiring the addition of auxiliary components orsubstances, but only modifying the concentration of a first substance inthe first liquid 11 of the sub phase.

The inventors have realized that at least for certain types of liquids11, 12, if the concentration of the first substance in the first liquid11 is too high initially, the desired phase separation may not occur,and the nanowires 20 may disperse in the first liquid 11. On the otherhand, if the concentration is too low, the nanowire aggregate 25 willnot be properly formed with densely packed nanowires 20. In oneembodiment, the first liquid 11 may initially contain said firstsubstance in a concentration exceeding a first level A, so as to obtainproper wetting characteristics of the wire portion 21 of the nanowires20 which preferably are coated 22. Dependent on specific choice of thefirst substance, the exceeded concentration level A may e.g. be 50%, ore.g. 70%, or e.g. 90%. In the step of modifying the composition of thefluid, the concentration of the first substance in the sub phase may bechanged so as not to exceed a second level B, which may be the same orlower than the first level A. Dependent on specific choice of the firstsubstance, the maximum concentration level B may e.g. be 50%, or e.g.30%, or e.g. 10%. This embodiment may be suitably employed where thefirst substance in the first liquid 11 phase decreases the density ofthe sub phase, e.g. where the first substance is isopropyl alcohol oracetone. The modification to a lower concentration thus increases thedensity of the sub phase, which may contribute to planarizing theinterface 13. The result of the modifying step may be seen in in theexemplary drawings of FIGS. 1C and 2B, respectively.

The inventors have also found the surprising effect that in variousembodiments it is possible to provide the fluid with a substance in acomposition configured to counteract bulging of the interface, bycareful selection of the fluid from the outset. In one embodiment, theaddition of hexane in the first liquid 11, in an aqueous solution of acertain concentration range, means that subsequent addition of thesecond liquid 12 of any of the aforementioned types may be accomplishedwhile still minimizing bulging of the resulting interface 13. In variousembodiments, hexane may be included in a concentration of 10-60%, suchas 10-40%, or even 10-30%, and preferably <30%, with a positiveplanarizing effect.

A benefit of such a substantially planar shape of the nanowireaggregate, with little or no bulging, is that it makes it possible totransfer the aggregate to larger substrate surfaces up to severaldecimeters or more than a meter wide, without adding defects such ascracks or holes that might appear due to the difference in curvaturebetween the aggregate of aligned NWs and the planar substrate, as willbe described below. In addition, the embodiments of planarizing theinterface 13 carrying the nanowire aggregate 25 by means of changing thecomposition of the fluid has the benefit of being very fast,substantially instantaneous, since it is a physical effect of thechanged composition.

In the overall method of transferring an assembly of oriented nanowiresfrom a fluid to a substrate surface, the step of bringing the nanowireaggregate 25 into contact with a substrate surface 31 is carried outsuch that a majority of the nanowires 20 are aligned with respect toeach other on the substrate 30. In one embodiment, the floatingaggregate 25 is captured using a suitable substrate, e.g. a piece of Siwafer. The substrate may be dipped into the fluid 10 close to theaggregate 25 at a certain angle, preferably not more than 30°, andcarefully lifted in order to capture the formed nanowire array. Once thearray 25 of vertically aligned nanowires 20 is captured, the sample,i.e. the substrate 30 provided with nanowires 20, may be kept inside adesiccator for some time in order to eliminate excess of solvent. In asubsequent step, the sample may be baked for an additional time toremove excess of the second liquid 12. After the baking step, the sampleis covered with a planarization compound, e.g. Level M10, for fillinggaps in between the standing nanowires 20 and creating a film where thenanowires are contained.

However, an alternative method for transferring the aggregate 25 to asubstrate 30 is provided here, which is particularly suitable for largersubstrates, e.g. larger than 2″, up to 6″ and more. Rather than“scooping” up the nanowire aggregate 25, a method of draining the subphase is employed, such that the floating aggregate 25 is controlled toland on the surface 31 of a substrate 30, placed at the bottom of thecontainer 1.

FIG. 6 schematically illustrates a container apparatus 60 fortransferring an assembly of oriented nanowires from a fluid to asubstrate surface in various embodiments. The apparatus 60 comprises avessel or container 1, and be made of any material suitable for holdingthe first 11 and second liquids 12 of a fluid 10 as disclosed. Thecontainer may thus have inner walls of e.g. glass, or a metal. A supportmember 2 may be provided at an inner bottom part of the container 1,having a substantially horizontal support surface 5 for supporting asubstrate 30. The support member may thus be planar or e.g. comprise anet structure or other shape, providing a substantially horizontalsupport surface 5. In one embodiment, the container apparatus 60 maycomprise at least one port or conduit 3, 4, at least for adding a liquidsubstance to the inner of the container 1. More particularly, said port3,4 is preferably provided below the substrate support surface 5 of thesupport member 2, such that liquid may be suitably injected to orextracted from a sub phase, as indicated in e.g. FIGS. 1 and 2. In oneembodiment, separate ports 3 and 4 may be provided for injection ofliquid to the sub phase, and for extraction of liquid from the subphase.

Example substrates that may be used include, but are not limited to,silicon, glass, plastic, molybdenum, silane modified silicon, gold,thiol modified gold or silicon surfaces with physically adsorbedcationic polymers. The substrate surface may be used as-received (i.e.bare), e.g. a clean Si wafer, which may comprise an oxide layer. In analternative embodiment, the surface of the substrate is functionalized.The functionalizing compound aids in securing the nanowires to thesubstrate surface. The surface of the substrate may be modified (e.g.functionalized) either by chemical reactions or physical adsorption of afunctional species that includes specific functional groups. Theassembly of nanowires may be transferred from the interface to thefunctionalized substrate surface as a result of electrostaticinteractions between the aligned nanowires and a functionalized surfaceor as a result of van der Waals interactions between the nanowires andthe substrate surface 31.

In one embodiment, a method may comprise the steps indicated in FIG. 5.

A first step 51 includes providing a fluid 10 to a container 1, saidfluid comprising a first liquid 11, a second liquid 12 and a pluralityof nanowires 20.

In a second step 53 the first 11 and second liquids 12 phase separatinginto a sub phase, a top phase, and an interface 13 between the sub phaseand the top phase.

In accordance with a third step 55, the nanowires are functionalized toalign vertically into a nanowire aggregate at the interface.

In a fourth step 57, a substance composition of the fluid is modifiedsuch that a curvature radius of the interface is increased towardsplanarization, in the sense that bulging of the interface iscounteracted. As noted above, in alternative embodiments the effect ofcounteracting bulging may be obtained by providing the fluid with thesubstance composition from the outset, e.g. in the first liquid prior toadding the plurality of nanowires. In such an embodiment, step 57 wouldnot be included as a separate step.

In an optional step 58, an additional amount of the second liquid may beadded to the top phase, such that a plurality of nanowire aggregates 25are interconnected into contiguous nanowire aggregate 25. Furthermore,this step may be carried out prior to step 57 in an alternativeembodiment.

In a step 59, the nanowire aggregate is brought into contact with asubstrate surface such that a majority of the nanowires are aligned withrespect to each other on the substrate.

Measurements carried out on substrate samples prepared with thesuggested method have shown very good results, and some test results areoutlined in Table 1 below. The numbers therein indicate that aperfection level obtained using this method for transferring aligned andoriented nanowires from a fluid to a substrate surface is unprecedented.

FIG. 8 shows an image of a sample of an assembly of nanowires 20 whichhave been transferred to a substrate 30, including one misalignednanowire 20 which illustrates the aspect ratio of the nanowires 20.

TABLE 1 Results of 246 measurements on 84 samples Alignment OrientationClose packing Mean 94.4 98.4 58.3 Std Dev 11.1 2.3 17.2 Std Err Mean0.7085 0.1482 1.0937 Upper 95% Mean 95.8 98.7 60.4 Lower 95% Mean 93.098.2 56.1 N 246 246 246

As can be seen from Table 1, 94.4% of the transferred nanowires arealigned within ±5 degrees from the normal direction. In addition, veryclose packing is obtained, where the number 58.3 indicates the surfacedensity divided by theoretical max density of circles with diameter ofthe nanowire 21, which is the brighter center portion. This means athicker shell or coating 22 will give a lower close packing value. Inaddition, the orientation obtained is 98.4%, meaning the relative numberof nanowires ordered in the correct direction. These measurements havebeen obtained from samples ranging from 1-6″.

In an embodiment, the substrate 30 with the captured assembly 25 ofnanowires 20 can be placed into a solar cell 71 if the nanowires 20 havea pn junction, as shown in FIG. 7. It should be noted, though, that thesubstrate 72 illustrated in FIG. 7 may be the same substrate 30 to whichthe nanowire aggregate 25 was transferred, or a later substrate 72connected to the opposing ends of the nanowires 20, after which theoriginal substrate 30 is removed. Alternatively, substrate 72 mayincorporate capture substrate 30, with additional layers or structuressubsequently provided.

As schematically illustrated in FIG. 7, the substrate 72 may containsemiconductor (e.g., GaAs, InP, etc.) nanowires 20 positionedsubstantially perpendicular (e.g., with the longest axis 80 to 100degrees, such as 90 degrees) to the upper substrate surface. Thenanowires 20 in this embodiment have an axial pn junction 21CC locatedbetween a lower first conductivity type (e.g., n or p type) segment 21Aand an upper second conductivity type (e.g., p or n type) segment 21B ofthe opposite conductivity type. In the solar cell 71, electrodes provideelectrical contact to the nanowires 20. For example, the solar cell 71may contain an upper electrode (e.g., transparent electrode) 73 inelectrical contact with the upper segment 21B of the nanowires and anelectrically conductive or semiconductor substrate 72 may provide anelectrical contact to the lower segment 21A of the nanowires 20. Aninsulating or encapsulating material 74 may be located between thenanowires 20. Alternatively, the nanowires may contain a radial ratherthan an axial pn junction, in which case segment 21B is formed as ashell surrounding a nanowire core 21A such that the pn junction extendssubstantially perpendicular to the substrate capture surface.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the invention is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the claims. All of the publications,patent applications and patents cited herein are incorporated herein byreference in their entirety.

1. A method for transferring an assembly of oriented nanowires from afluid to a substrate surface, comprising: providing a fluid to acontainer, said fluid comprising a first liquid, a second liquid and aplurality of nanowires, wherein the first and second liquids phaseseparate into a sub phase, a top phase, and an interface between the subphase and the top phase; wherein the nanowires are functionalized toalign vertically into a nanowire aggregate at the interface; whereinsaid fluid is provided with a substance in a composition configured tocounteract bulging of the interface; and bringing the nanowire aggregateinto contact with a substrate surface such that a majority of thenanowires are aligned with respect to each other on the substrate. 2.The method of claim 1, wherein the nanowires are provided in the secondliquid, prior to combining the second liquid with the first liquid. 3.The method of claim 1, further comprising the step of adding asubsequent amount of the second liquid to the top phase, such that aplurality of nanowire aggregates are interconnected into a largercontiguous nanowire aggregate.
 4. The method of claim 1, wherein thesubstance composition increases the relative density of the sub phasewith respect to the top phase.
 5. The method of claim 1, comprisingchanging the composition of the sub phase subsequent to forming thenanowire aggregate.
 6. The method of claim 5, wherein the composition ofthe sub phase is changed so as to increase its density.
 7. The method ofclaim 5, wherein the composition of the sub phase is changed byextracting an amount of the first liquid from the sub phase, and addingan amount of liquid to the sub phase, wherein the added amount of liquidhas a different composition than the extracted amount of liquid.
 8. Themethod of claim 5, wherein changing the composition of the sub phaseincludes providing the first liquid to the sub phase with a firstconcentration of said substance exceeding a first level prior toproviding the nanowires to the fluid, and changing the substanceconcentration of the sub phase to a second concentration below a secondlevel, which is lower than the first level, after forming the nanowireaggregate.
 9. The method of claim 1, wherein said substance includes atleast one of acetone, acetonitrile, dimethyl sulfoxide, di ethyleneglycol, dioxane, dimethoxyethane, dimethylformamide, tert-butyl alcohol,2-propanol and isopropyl alcohol, which substance is provided in thefirst liquid in a composition with a solvent at a predeterminedconcentration range.
 10. The method of claim 1, wherein said substanceincludes hexane provided in the first liquid in a composition with asolvent at a predetermined concentration range.
 11. The method of claim9, wherein said substance is provided to said concentration in water.12. The method of claim 1, wherein the second liquid has a compositioncomprising a concentration of a substance of at least one of toluene,hexane, octane, and cyclohexane.
 13. The method of claim 1, wherein thenanowires are provided with a metal catalyst particle at one end, andare functionalized by means of a compound comprising a molecule chainconnecting to the catalyst particle.
 14. The method of claim 13, whereinthe molecule chain is a thiol connecting by means of a sulfur atom tothe catalyst particle.
 15. The method of claim 13, wherein the compoundis 1-octadecanethiol or polyethyleneimine.
 16. The method of claim 13,wherein the nanowires are functionalized with a compound of the secondliquid.
 17. The method of claim 13, wherein the nanowire has adielectric surface coating.
 18. The method of claim 17, wherein thedielectric surface coating comprises silica or aluminum oxide.
 19. Themethod of claim 1, further comprising providing the substrate in thecontainer with the substrate surface substantially horizontal; whereinthe step of bringing the nanowire aggregate into contact with asubstrate surface comprises extracting liquid from the sub phase untilthe nanowire aggregate engages the substrate surface.
 20. The method ofclaim 19, wherein the substrate is provided on a support member in thecontainer, and the liquid of the sub phase is extracted through aconduit connected to the container below the substrate surface.